Analytical Spectroscopy and ImagingRecognizing that Raman Spectroscopy is an important analytical spectroscopic technique, Prof. Cullum has assembled three Raman spectrometers, each using CCD detection technology. Two steady-state ion laser excitation sources and a pulsed YAG pumped OPA laser system are used as scattering sources to allow investigators to select the wavelength best suited for a specific application. Solids, liquids, solutions, and other complex media are routinely examined. Fiber optics, tipped with thin metal coatings to take advantage of the surface enhanced Raman effect (SERS), are used to probe sub-micron chemical and biological systems.
The high intensity output of pulsed laser systems can be used in a wide array of non-linear optical imaging applications, including multi-photon photoacoustic imaging. Near-infrared radiation passes harmlessly into living tissue to great depth. Using multiphoton absorption of specifically designed dyes, subcutaneous structures, including cancerous tumors, can be imaged. These high intensity pulses can also be used in coherent back-scattered spectroscopy to remotely detect and identify airborne materials in the atmosphere. This technique is being developed to use temporal focusing of ultra-short laser pulses to allow characterization of air samples at distances of hundreds of meters to several kilometers away.
DynamicsOur laser facility is unique within Maryland in that we have the capability of studying chemical and biological processes that occur on any significant timescale. Processes can be studied using timescales as long as months or as short as femtoseconds. Stopped-flow techniques allow relatively slow processes to be observed using a variety of detection scenarios, including absorption and emission. An ultra-fast Ti-Sapphire laser system allows our students to examine the most fundamental chemical transformations as they occur. Included within the facility are nanosecond time-resolved absorption and emission techniques that allow bimolecular reactivity of chemically and biologically significant systems. The mechanisms of DNA photodamage and electron transfer reactivity are examples of the studies of current interest. Picosecond pump-probe linear dichroism spectroscopy is used to study unimolecular processes including electron transfer, rotational dynamics and topochemical transformations. A Ti-Sapphire laser, coupled with a time-correlated single photon counting apparatus, rounds out our facility to allow our students to examine the dynamics of fluorescent systems. The apparatus is equipped with a state-of-the-art microchannel plate detector to provide time resolution down to 25 ps. Of course, included in our shared instrument laboratory are research grade uv-visible and fluorescence spectrometers, HPLC and GC systems and a photochemical reactor to allow characterization of samples before study and the analysis to photochemical reaction products.